Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the ion beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity.
Two concerns of the solar cell manufacturing industry are manufacturing throughput and cell efficiency. Cell efficiency measures the amount of energy converted into electricity. Higher cell efficiencies may be needed to stay competitive in the solar cell manufacturing industry. However, manufacturing throughput cannot be sacrificed at the expense of increased cell efficiency.
Ion implantation has been demonstrated as a viable method to dope solar cells. Use of ion implantation removes process steps needed for existing technology, such as diffusion furnaces. For example, a laser edge isolation step may be removed if ion implantation is used instead of furnace diffusion because ion implantation will only dope the desired surface. Besides removal of process steps, higher cell efficiencies have been demonstrated using ion implantation. Ion implantation also offers the ability to perform a blanket implant of an entire surface of a solar cell or a selective (or patterned) implant of only part of the solar cell. Selective implantation at high throughputs using ion implantation avoids the costly and time-consuming lithography or patterning steps used for furnace diffusion. Selective implantation also enables new solar cell designs. Any improvement to manufacturing throughput of an ion implanter or its reliability would be beneficial to solar cell manufacturers worldwide. This may accelerate the adoption of solar cells as an alternative energy source.
“Glitches” may occur during the ion implantation process. A glitch is defined as a sudden degradation in the beam quality during an ion implantation operation, typically due to a variation in an operating voltage. Such a glitch is typically caused by interactions between components along the beam path, which affect one or more operating voltages and can be caused at various locations along the beam path. For example, ion implanters generally employ several electrodes along this beam path, which accelerate the beam, decelerate the beam, or suppress spurious streams of electrons that are generated during operation. Each of these electrodes is maintained at a predetermined voltage. Often, electrodes of different voltage are located near each other and therefore arcing may occur between electrodes. Generally, arcing occurs across acceleration gaps, deceleration gaps, or suppression gaps, although arcing may occur elsewhere. Interaction between, for example, a source extraction voltage, source suppression voltage, and source beam current may cause a glitch. These glitches may be detected as a sharp change in the current from one of the power supplies. If the implantation is interrupted or affected by a glitch, the implanted solar cell or other workpiece may be negatively affected or even potentially rendered unusable. For example, a solar cell may have a lower efficiency due to the lower implanted dose caused by a glitch.
Use of a fluoride-containing gas during implantation may limit throughput due to this glitching. With a fluoride-containing gas, such as BF3, this glitching may include arcing at the various electrodes in the implanter, such as between the ion source and the extraction electrodes. Any method that reduces glitching in an ion implanter will increase throughput and improve the quality of the implanted workpieces.